A Rare Case of a Primary Cutaneous Collision Tumor Comprising Malignant Melanoma and Rhabdomyosarcoma.

This case reports a 35-year-old man who presented with a painful erythematous nodule on his right posterior calf. He first noticed this nodule several years ago and it often bled upon contact with clothing. An excisional biopsy of the skin lesion revealed two distinct populations of cells. One population of epithelioid cells stained positive for Mart-1, HMB45, and SOX-10, confirming the diagnosis of malignant melanoma. The second population of cells stained positive for desmin and calponin, confirming the diagnosis of sarcoma with muscular differentiation. Subsequently, these unusual findings led to the diagnosis of a collision tumor comprising malignant melanoma and rhabdomyosarcoma. Follow-up PET/CT and brain MRI revealed no metastasis from the primary skin lesion. This case highlights a rare combination of cell types found within a collision tumor in addition to providing details on how to diagnose this skin lesion.